1. Field of the Invention
This invention relates to a printer for recording images by an electrophotographic process. This invention particularly relates to an electrophotographic printer for recording high-gradation, continuous tone images.
2. Description of the Prior Art
There have heretofore been known electrophotographic printers for forming an electrostatic latent image by exposing a uniformly charged photoconductor to a light beam modulated based on image signals representing a continuous tone image, and then forming a hard copy of the image by the ordinary electrophotographic process.
On the other hand, various electrophotographic processes for recording a continuous tone image with good gradation reproducibility have heretofore been proposed. For example, in Japanese Patent Publication No. 49(1974)-38172, it has been proposed to employ an electrophotographic process wherein an electrostatic latent image formed on a photoconductor is developed into a toner image by use of a liquid toner, an adhesive tape is then closely contacted with the toner image under pressure to peel off and pick up the toner image onto the adhesive tape, and thereafter the adhesive tape is adhered to a final substrate.
The liquid toner used in the proposed electrophotographic process comprises an insulating liquid and minute charged toner particles dispersed therein. The toner particle sizes are normally within the range of 0.1 .mu.m to 1.0 .mu.m, and thus are smaller than these of the dry toner particles which are used in a dry developing process. Therefore, the proposed electrophotographic process is advantageous for the recording of a continuous tone image.
As to the method of transferring the toner image formed on the photoconductor onto a substrate such as paper, the method wherein the toner image is electrostatically transferred by use of "corotoron charger" has been used widely. However, this transfer method has the drawback that the transfer efficiency is low both at high density regions and at low density regions. On the other hand, with the adhesive transfer method wherein the toner image is transferred to the adhesive layer in the manner mentioned above, a very high transfer efficiency can be obtained regardless of the image density. Also from this viewpoint, the proposed electrophotographic process mentioned above is advantageous for the recording of a continuous tone image.
In cases where the adhesive transfer is carried out as mentioned above, a sheet provided with an adhesive layer which is adhesive at room temperature or a sheet provided with a thermally-sensitive adhesive layer which is not adhesive at room temperature and becomes adhesive upon being heated is used as the transfer sheet. The former transfer sheet is processed in a form in which it is provided with release agent coated paper. Therefore, in the case where the former transfer sheet is used, a means for removing the release agent coated paper from the transfer sheet prior to the adhesive transfer step, a means for winding up the release paper removed from the transfer paper or the like is necessary. On the other hand, the latter transfer sheet requires no such means, and therefore is advantageous from the viewpoint of making the printer smaller and lighter.
However, it has been usually often recognized that, even though the wet developing process and the adhesive transfer process as mentioned above are employed, gradation reproducibility deteriorates when the adhesive transfer is carried out with usual adhesive tape to record a high-gradation, continuous tone image.